In internal combustion engines which work on the piston engine principle, the sliding movement of the piston and of the piston rings, located in the piston grooves, which ensure that the system is sealed in such a way that the gases from combustion do not get past the piston into the crankcase, leads to wear of the friction partners involved. These are mainly the cylinder and the piston rings which slide on it.
In particular, the first piston ring positioned closest to the combustion chamber is subjected to very high thermal load stresses and combustion pressures. Above all, this leads to an increased corrosion and an increased wear on the flank surface, facing towards the combustion chamber, of the first piston ring. A further problem for the first piston ring is that this ring must run under the worst lubrication conditions. In order to meet these extreme requirements, the flank surfaces can be hardened for an increased wear resistance. Wear-prevention coatings, for example of hard chromium, can be applied to the outer circumferential surface of piston rings, which functions as a running surface.
A piston ring is described in DE 10 2005 041 408 A1, the surface of which is provided with a hardened layer, wherein a nitriding or chrome nitriding process can be used for the hardening, and to the flank surfaces or the outer circumferential surface of which a wear-prevention layer can be applied.
Such piston rings have an increased hardness on the nitrided flank surfaces. Nitrided surfaces, however, have the disadvantage of only a low corrosion resistance, because an oxide protection layer can no longer form in a natural way due to the bonding of the passivating elements. The corrosion resistance of the upper flank surface and the wear resistance of the outer circumferential surface of such piston rings are therefore still in need of improvement.